In the printing industry, graphical information such as photographs or artwork can be reproduced by one of several types of printing process. Lithographic printing is one such printing process. In lithographic printing, a lithographic plate is mounted on a lithographic press. The lithographic plate includes a hydrophilic surface on which an image pattern is created using hydrophobic material. A hydrophobic ink is used in printing. The ink is attracted to the hydrophobic image area on the plate and is repelled by the hydrophilic non-imaged area on the lithographic printing plate. The inked image is then used for making lithographic prints. The lithographic printing process is a complex process involving wet chemicals and costly equipment.
An ink jet printer produces images on a receiver medium by ejecting ink droplets onto the receiver medium in an imagewise fashion. The advantages of non-impact, low-noise, low energy use, and low-equipment cost in ink jet printing are not only responsible for the wide acceptance of ink jet printers homes and offices, but are also appealing to printing and publishing applications, especially in the context of digitally processed printing plates.
U.S. Pat. No. 4,833,486 disclosed an ink jet image transfer lithographic apparatus that transfers hydrophobic solid ink onto a lithographic plate in an image pattern according to the graphics and textual information. The plate can then be mounted in a lithographic press for lithographic printing. The printing image pattern can also be formed directly on the plate cylinder of the lithographic press.
In the printing industry, a digital halftone image typically comprises a plurality of screen dots as the basic image pixels with each screen dot providing gray scale in the image. The resolution of the screen dots, or the screen ruling, can change from 75 screen dots per inch to provide the lowest image quality to 300 screen dots per inch for the highest image quality. Each screen dot comprises a matrix of n.times.n micropixels with n typically being in the range of 1 to 12. Each of the micropixels can be occupied by a microdot. (see FIGS. 4 and 5).
Several requirements therefore exist for digitally reproducing a halftone image on a lithographic printing plate. The screen dot size (i.e. the screen ruling) needs to be adjusted to accommodate different printing resolutions as required by the printing jobs while the number of the micropixels within each screen dot may be kept the same. Furthermore, for a fixed screen ruling, the areas of the halftone dots need to be varied to simulate tone scale in an input image (i.e. area modulation). The size of the halftone dots can vary from one microdot to the full coverage of the screen dot (i.e. n.times.n microdots).